A et al. (2005, 2007) and related papers on mating patterns and gene dispersal in Prunus mahaleb

Estacio^´n Biolo^´gica de Don˜ ana, Consejo Superior de Investigaciones Cient^´ıficas (CSIC), Avenida Ame^´rico Vespuccio s ⁄ n, E-41092 Sevilla, Spain

19 May 2009

Parentage analysis of progeny arrays based on highly- variable molecular markers has substantially contributed to our understanding of mating systems and gene dispersal patterns in wild populations of plants (Jones & Ardren

2003). One of the methods commonly used, known as parentage analysis, involves genotyping samples of seeds or seedlings produced by maternal parents of known or

Correspondence: Carlos M. Herrera, Fax: + 34 954 62 11 25; E-mail: herrera@cica.es

unknown genotypes, and then probabilistically inferring the identity of paternal parents by the application of several statistical procedures (Jones & Ardren 2003; Kalinowski et al. 2007). For instance, paternity analysis to infer the con- temporary dispersal of pollen requires that the genotypes of all or most individuals potentially siring the offspring of focal maternal parents are known, or at least a good estimate of the proportion of candidate parents sampled. As shown by recent studies, this crucial requirement is most easily met in discrete, relatively small populations that are spatially isolated from other conspecific populations. In such situations, the true fathers of the offspring from mater- nal families can be identified exactly (Krauss 1999; Garc^´ıa et al. 2005, 2007; Robledo-Arnuncio & Gil 2005; Smouse & Robledo-Arnuncio 2005; Robledo-Arnuncio & Garc^´ıa 2007).

Garc´ıa et al. (2005, 2007) used microsatellite markers and parentage analysis to study the mating patterns and the spatial components of pollen and seed dispersal in a group of 196 individuals of the insect-pollinated, vertebrate- dispersed, gynodioecious tree Prunus mahaleb (Rosaceae). Garc^´ıa et al. (2005: 1822–1823) described their study trees as a ‘highly isolated population where all trees have been sampled’, also highlighting that ‘this represents a thorough sample of the whole population and we are confident that it includes all the reproductive trees’. They also noted that the nearest P. mahaleb population in the study region was located at 1.5 km distance and that the area had been exhaustively sampled. Garc^´ıa et al. (2007: 1948) provided a similar description of the study system, likewise depicting the stand of trees studied as an isolated population sepa- rated from the nearest other population by 1.5 km.

I found that Garc´ıa et al.’s (2005, 2007) depiction of their study system bears little resemblance to reality and that their study trees do not form a discrete, geographically iso- lated population. As shown in Fig. 1, the 196 P. mahaleb trees studied by Garc^´ıa and colleagues, far from being a geographically isolated group, are part of a larger popula- tion extending in a southwest-northeast direction for about

2 km. Around 300 adult reproductive trees occur within the 1.5 km distributional gap proclaimed by Garc^´ıa and colleagues (Fig. 1). In August 2008, when I found and mapped these additional trees, all of them bore fruits, empty fruit pedicels, and ⁄ or inflorescence remains, thus attesting their reproductive status. Furthermore, the heights (>3 m) and trunk diameters (mostly between 20–40 cm) of these trees were essentially the same as those of the trees studied by Garc^´ıa et al. (2005, 2007), which points to their similarity in terms of age and, presumably, popula- tion history (see Appendix S1).

Did the ignoring of this large number of ungenotyped parents in the immediate neighbourhood of genotyped focal trees affect the main conclusions of Garc´ıa et al.

2007) in the Sierra de Cazorla, southeastern Spain. Blue dots represent the locations of the trees studied by the above authors (N = 196; taken from Fig. 1 in Garc´ıa et al. 2007); red dots are the N = 391 additional adult trees found in the area (geographical coordinates of these additional trees are available in Appendix S1).

(2005, 2007)? Only corresponding studies applying exhaustive sampling and genotyping of all the additional trees in the area could answer this question with cer- tainty. Even in the absence of such an experiment, some consequences can be anticipated, for it is well known that exhaustive sampling of potential parental genotypes is essential for the reliability of paternity, maternity or par- entage studies (Jones & Ardren 2003; Robledo-Arnuncio & Gil 2005; Smouse & Robledo-Arnuncio 2005; Robledo- Arnuncio & Garc^´ıa 2007). The presence of a significant number of ungenotyped reproductive trees in the neigh- bourhood of focal ones can have at least the following two major implications for the results of Garc^´ıa et al. (2005, 2007).

Firstly, the fraction of progeny for which true parents were assigned was likely overestimated or, to put it in another way, a certain proportion of parental assign- ments were probably erroneous. Such an effect could be quantitatively important, as suggested by simple simula- tions of paternity analyses performed using the program CERVUS 3.0.3 (Kalinowski et al. 2007). Using the allelic fre- quencies for nine microsatellite loci in the example file

furnished with the program, I simulated father assign- ment rates given a known mother for two situations: (a) sampling 104 (95%) of 109 trees, i.e. a situation akin to that purported in Garc´ıa et al. (2005, 2007), treating their

104 hermaphrodite trees as the only potential pollen donors; and (b) sampling 104 (33.3%) of 312 trees (i.e.

587 total trees in the area as given in Fig. 1 · 0.531 her- maphrodite fraction). The expected proportion of unas- signed progeny rose from 3% in case (a) to 63% in case (b). The rise in the uncertainty in parental assignment was even greater for estimates without information on maternal genotypes (from 4% to 88%), as is the case in parentage analysis of dispersed seeds with unknown maternal parents. A second likely consequence, stemming from both the uncertainty in parental assignments and the spatial proximity of many ungenotyped potential par- ents, is that rates of pollen and seed immigration into the studied population (i.e. long-distance gene immigra- tion), and the distances over which gene flow occurred, were probably overestimated. For example, Garc´ıa et al. (2007) estimated that around 20% of dispersed seeds were derived from other populations more than 1.5 km away, because their presumed parental genotypes did not match any of the genotyped focal trees. Many of these presumed long-range immigrants, however, might have been the progeny of nearby-growing but ungeno- typed parents (Fig. 1).

The potential consequences of the two above described effects should also be kept in mind in relation to several related studies likewise based on parentage analysis and conducted on the same stand of P. mahaleb trees and, apparently, the same seed progenies as studied by Garc^´ıa et al. (2005, 2007). Godoy & Jordano (2001), Jordano et al. (2007), Robledo-Arnuncio & Garc^´ıa (2007) and Fortuna et al. (2008) all assumed spatial isolation and exhaustive genotyping of potential parents. As these premises have been proven here to be false, the conclusions of these studies with respect to mating patterns and pollen and seed dispersal should also be interpreted with caution. For example, estimates of seed immigration rates from other populations presented by Godoy & Jordano (2001) and Jordano et al. (2007) are probably inflated by the large number of ungenotyped trees in the immediate neighbourhood. Likewise, the spatial organization of pair- wise mating events (‘spatial mating networks’) reported by Fortuna et al.’s (2008) might be severely distorted by erroneous or uncertain parentage assignments. From all the preceding considerations it appears that a ‘major remaining challenge in parentage analysis is to obtain appropriate and complete field samples’ and ‘that more effort should be devoted to creative sampling practices (i.e. spend more time in the field)’ (Jones & Ardren 2003:

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Godoy JA, Jordano P (2001) Seed dispersal by animals: exact iden- tification of source trees with endocarp DNA microsatellites. Molecular Ecology, 10, 2275–2283.

Jones AG, Ardren WR (2003) Methods of parentage analysis in nat- ural populations. Molecular Ecology, 12, 2511–2523.

Jordano P, Garc´ıa C, Godoy JA, Garc´ıa-Castan˜ o JL (2007) Differen- tial contribution of frugivores to complex seed dispersal patterns. Proceedings of the National Academy of Sciences USA, 104, 3278–

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Robledo-Arnuncio JJ, Gil L (2005) Patterns of pollen dispersal in a small population of Pinus sylvestris L. revealed by total-exclusion paternity analysis. Heredity, 94, 13–22.

Smouse PE, Robledo-Arnuncio JJ (2005) Measuring the genetic structure of the pollen pool as the probability of paternal iden- tity. Heredity, 94, 640–649.

Supporting information